RFR: 7903723: Jextract binary should live in a bin folder

[Maurizio Cimadamore]

This PR reintroduces the `bin` folder for jextract launcher scripts.

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Commit messages:
 - Initial push

Changes: https://git.openjdk.org/jextract/pull/242/files
  Webrev: https://webrevs.openjdk.org/?repo=jextract&pr=242&range=00
  Issue: https://bugs.openjdk.org/browse/CODETOOLS-7903723
  Stats: 11 lines in 3 files changed: 3 ins; 0 del; 8 mod
  Patch: https://git.openjdk.org/jextract/pull/242.diff
  Fetch: git fetch https://git.openjdk.org/jextract.git pull/242/head:pull/242

PR: https://git.openjdk.org/jextract/pull/242